Fix infinite loop in example under Section 2.6.2 Loops over list of items

Author: hansonchar

doc/latex/pgfplots/TeX-programming-notes.tex

@@ -926,7 +926,7 @@ \subsubsection{Loops over list of items}
 }%
 \long\def\looppicker#1#2#3,{%
 	\def\tempitem{#3}%
-	\ifx\tempitem\listingloopENDMARKER
+	\expandafter\ifx\tempitem\listingloopENDMARKER
 		\let\next=\relax%
 	\else
 		\def#1{#3}%